Mud! Why should you put Mud, of all things, in a coral reef aquarium? Well, the simple answer is that that mud will help create an environment that will almost force your corals and other decorative animals to thrive. With some 35 years of experience as a marine ecologist behind me, I can say that THE most important component of a coral reef aquarium is a deep sand bed, comprised of very fine sandy sediments that we can, without any hesitation, call MUD. In this article, I will discuss three things, first, the benefits of a sand bed, then how to set a bed up and, finally, some of the possible problems that you might encounter.

Benefits:

Hobbyists might think that sand beds have no place in a coral reef aquarium, particularly if they are trying to establish something resembling a natural coral reef. However, with some thought I am sure they would realize that most coral reefs are surrounded by sand area, and by constructing a sand bed in our aquariums we merely emulate nature. These beds provide three things. First, they provide a place for processing and exporting some dissolved nutrients. Second, they provide a place to recycle detritus, excess foods, animal feces and other particulate material into useable forms. Finally, they provide a food source for many reef animals. Let's look at each of these functions.

As they do in nature, the sand grain surfaces of sand beds in our systems provide the major substrate for nutrient-processing bacteria. The bacterial population is determined by three factors: the total sand surface area; the amount of nutrient available; and the number and effects of bacterial predators. All of these play a role in the development of the sand bed biological filter.

In a given volume of sand, the usable bacterial surface area rises rapidly as the average particle size decreases. For example, a cubical particle 1 mm on a side has 6 square mm of surface area, while the surface area on a particle that is one eighth (or 0.125) mm on a side is a total of 0.09375 square mm. However, in the volume of 1 cubic mm, there would be 512 of the smaller particles, for a total area of 48 square mm, eight times what is found on the larger cube.

The total sediment surface area in even a small tank is impressive, indeed. In my 45 gallon reef tank, the sand bed averages about 4 inches deep, by 12 inches wide, by 36 inches long, for a total of one cubic ft of sediment. I won't bore you with the calculations, but if the average particle size is one eighth mm, and that is a good average size to have, the total sand surface area is about 14,828 square feet or just slightly over 1/3 of an acre. A LOT of bacteria can live with that amount of space!

Although we seldom consider bacteria when we set up our systems, they are exceptionally important to the survival of every decorative fish or coral we add to the tank. Those bacteria are the biological filter of your reef tank, and by their simple existence and growth they detoxify and remove many of the excess nutrients from the system.

One organism's poison is another's nutrient. Fish and invertebrate urine, largely ammonium hydroxide, or ammonia gas dissolved in water, is the primary byproduct of necessary protein metabolism. Ammonia gas, even very small amounts dissolved in water, is highly toxic to animals. Likewise, phosphates are also byproducts of animal metabolism, and although not toxic to most animals, high phosphate concentrations may reduce or stop coral growth. The removal of both nitrogenous wastes, such as ammonia, and phosphates is accomplished by bacteria and microalgae which absorb these toxic animal byproducts and use them in their growth as necessary, required, and vital nutrients.

The surface area for bacteria and microalgae in live rock or on other surfaces is insignificant compared to the area in a sand bed four or more inches in depth. The cardinal rule of animal husbandry is that you have to feed animals, and many reef animals need to eat a lot. My article in the February 2001 Aquarium Fish Magazine (online version available: here ) about the composition of many foods and additives can be used to calculate just how much of the various nutrients you add to your system. As an average the dried foods that I tested had about one half of their weight as protein, which in turn means they have a very large amount of phosphate in them. And, if that was not enough, once the food has been eaten and processed by the animals, they urinate out protein byproducts as ammonia. Simply feeding your fish or corals the necessary food they need to live may boost ammonia and phosphate concentrations several hundred to several thousand times what is normally found in reef water. But, if you have a deep sand bed, a process that is nothing short of miraculous occurs. The bacteria and algae living in the sediments take up the nutrients so fast and so thoroughly, that hobbyist test kits typically may not measure any of the nutrients at all even immediately after feeding.

These nutrients act as food for the bacteria. In a very real sense, the biological filter depends upon bacterial growth. The breakdown of nitrogen compounds to nitrogen gas is done by bacteria growing in the areas of lowered oxygen concentration in the deeper parts of the sediments. At normal reef temperatures, around 82 deg F, some bacterial species will double their population in less than a half hour if they have the appropriate nutrients. This rapid bacterial growth rate causes the release of nitrogen gas which becomes visible as bubbles in the sediments.

Left. Sediments in my 45 gallon lagoonal reef tank showing gas (Nitrogen) bubbles in the sediments; several sand layers determined by oxygen concentration are evident. The bed is about four inches deep. Center. The same tank, several months earlier. Note the coarse material, the GARF grundge, to the right on the sediment surface. The large particles in this acted to reduce the worm access to surface and eventually caused sediment clumping to occur. Upon removal of the large fragments, the clumping disappeared. Right. The front of the same tank. Note the worm tubes extending from the surface of the sediment through the oxygenated layer. Movement of worms in these tubes pumps water into the lower levels preventing them from becoming completely anaerobic, and facilitating the biological filter.

Rapid bacterial growth rates only occur without competition for space or nutrients. As the bacterial populations fill in all the open spaces growth slows and may stop altogether. Some bacteria also secrete a exterior covering called a "glycocalyx." These glycocalices are made of a hard sugar-like material similar in consistency to rock candy. Rapid bacterial growth may produce so enough of this material to glue sediments together. These sediment lumps may be glued so tightly together that hammering is needed to break them apart. In much reef literature, these lumps are said to be caused by calcium carbonate or calcium phosphate precipitation. Such mineral precipitation is rare; if a small sediment lump is placed in a weak solution of household chlorine bleach, it breaks down to the component sediment grains in a short time. If the lumps were formed from the calcium salts, they would not dissociate in the bleach.

Lump formation is a disaster for the biological filter. The lumps restrict water flow and trap organic material where it can rot. Additionally, lump formation shuts down the biological filter by covering the bacteria and preventing them from metabolizing nutrients. This, in turn, causes the tank nutrient levels to skyrocket.

Fortunately, prevention of sediment clumping and the simultaneous maintenance of optimal biological filter operation is easily done by the establishment of a healthy and diverse sediment dwelling fauna, or "infauna." The infauna, so-called as the FAUNA lives IN the sediments, is a very diverse array group of wonder-working organisms. Unfortunately, they are small, and are not particularly attractive. Like Rodney Dangerfield, "They don't get no respect." And, that is a pity, as they do most of the work in keeping any reef tank functional.

The infauna are "the clean-up crew" and the "reef-janitorial" staff, and the array found in a successful tank may be DIVERSE! More than 200 different species commonly are found living in a mature sand bed. These include many types of flatworms, round worms, dozens of species of bristle worms, small snails, brittle stars, small sea cucumbers, protozoans, and many types of small crustaceans. The total populations may be immense. I have done sampling to measure the abundances found in the 45 gallon tank I mentioned earlier, and the number of animals larger than half a mm, or about one fiftieth of inch, in that tank ranges from 90,000 to 150,000 depending on what part of their population cycle the various species are in.

Left. An harpacticoid copepod, about 1/50th of an inch long. Barely visible, these small crustaceans are an important part of the food chains and clean-up crews in our tanks. They live on and in the sediments. Center. A group of tube-dwelling bristle worms, probably chaetopterids, in my 60 gallon Stichodactyla tank. These animals are primarily filter feeders catching small particles with their paired feeding tentacles. Left. The head end of a small predatory bristle worm called a syllid. These probably eat other small worms and move through the sediments in search of them. This worm was about 1/10th of an inch long.

What does this diverse and abundant array of critters do for and in the sand bed? Well, some will eat excess food, detritus, or algae. In doing so, they utilize it, and excrete part of it as waste. In turn, bacteria utilize that, and thus the infauna help keep the biological filter going. Additionally, many infaunal animals burrow ingesting some sediments as they go. They digest the microorganisms off of them, opening space for bacteria to grow.

By moving through sediments, the animals jostle and move the particles. Not much, just a little tiny bit. It has been estimated that each day each small organism moves about 10 to 100 cubic millimeter of sediment. Multiplying this tiny average amount of jostling by the number of animals in the tank gives the total amount of disturbance. In my 45 gallon tank, with an average population of about 100,000 small animals, from one to ten million cubic millimeters of sediment is moved each day! Or phrased another way, the entire tank's sediment volume could be completely turned over at least once every three to thirty days. With this amount of jostling and sediment eating, sediment clumping the sediments will simply not occur.

Consequently, excess food is eaten and disposed of or recycled as animal or algal flesh, and that the biological filter is maintained in the best of condition. And, best of all you, as the aquarist, didn't have to do anything. The animals did it all for you. All you had to do was to sit back, and enjoy a healthy tank. And, yes, I know it was a dirty job, but somebody had to do it...

A baby fire worm found in the sediment bed of my 45 gallon lagoonal reef tank. Juvenile worms such as this are commonly produced by spawning adult worms, and have passed through a relatively long planktonic larval stage in the tank water. Most of the babies perish by being eaten by corals and other suspension feeders.

But this isn't all the good a sand bed will do for your system! Most of the infauna live a year or less. However, they grow rapidly and start reproducing within a few weeks after they were spawned. Cumulatively, they produce large amounts of small eggs, sperm, and larvae that are liberated invisibly into the tank's water. All the spawned material has the potential of becoming food for many small-polyped stony corals as well other filter feeders. It is no coincidence that, historically, aquarists began to be able to keep many of these small polyped corals when they started keeping a sand bed in the aquarium for the first time.

Construction:

Making a sand bed is almost too easy. The most important part of the sand bed is, not surprisingly, the sand. While earlier I referred to "mud" and now I refer to "sand," I am not discussing two different materials. There is no scientific definition of "mud," however, those of us befuddled folks who spend part of our life working with marine sediments have a naming scale for the parts of the continuum of particles ranging from the very big ("boulders" = particles over 25.6 cm, about 10 inches, diameter) to the very small ("clay" = particles less than 0.004 mm, about 0.00016 inches). Nowhere in this scale is there a mention of that most desirable of substances, "mud." Generally, what a sediment-studying scientist would refer to as fine or very fine sands with smidgen of silt, most normal folks call mud. These are sediments whose particles generally range from about 1/16th mm (0.063mm) to about 1/8th mm (0.125 mm).

What's all the fuss about sediment grain size, anyway?

In all of my discussions about sand beds I have made a point of specifying one particular parameter, that of the average size of sediment particles in the sand bed. Why should this one factor be so important? The answer simply is that sediment particle sizes determine the acceptability of the sediment to the organisms. Perhaps an example might illustrate this statement better. One of the common amphipods found along the west coast of North America is a species called Rhepoxynius abronius . This small bug has been investigated in some detail as an organism to use to test the toxicity of sediments, has been found to prefer sediments of a specific particle size, 0.113 mm in diameter. If given a choice, it will move to and live in sediments of that one specific size, not sediments 0.110 mm nor sediments of 0.115 mm, but only of that one size. If individuals are experimentally confined to other sediment sizes, they neither live as long, nor reproduce as well, nor tolerate stressful conditions as do individuals kept at the optimum grain size (Ott, 1986).

Most sediment-dwelling organisms appear to have similar precise preferences. However, most will also live at least marginally well in mixed-sediments with sizes around their optima, and most sediment particle size optima seem to be in the range of 0.050 to 0.200. Consequently I suggest a range averaging about 0.125 as a good compromise. It isn't specifically the best for most infaunal species, but it will allow a diversity of species to live pretty well.

A good sediment particle size distribution for a sand bed.

Coarser sediments such as gravel or crushed coral are simply too big. Additionally, they have the drawback of being sharp edges that are abrasive to many of the small crustaceans and worms that must crawl through the sediments. Finer sediments can pack so tightly together that they are impervious to most animal movement, creating a layer that restricts animal and water flow shutting down the biological filter.

Having to assess sands for particle sizes would be a daunting task for any hobbyist. Fortunately, however, several vendors sell bulk sands in the appropriate size ranges, often marketed as "sugar fine" or oolitic sands. A few larger particles in the sediment mix is okay, but larger sediments should not constitute more than about 15 percent of the total. Under NO circumstances should you use crushed coral or coral gravel. These substrates are too coarse and often too abrasive for many of the smaller organisms to survive in.

The organisms do not care about the sediment mineral composition, only the particle sizes and shapes. Most aquarists use the commonly available aragonitic sands to "provide a calcium reservoir." Additionally their bright white color is often aesthetically pleasing. However, if the system's pH and calcium concentrations get low enough to dissolve significant amounts of the sediment there are some very severe problems and all the sand in the world won't help. Very successful tanks may be set up utilizing black lava sand, or fine siliceous sand, as long as the grains are of the appropriate size. There is some concern that siliceous sand will fuel diatom blooms, but such blooms may be controlled by the appropriate grazers. There is absolutely no need for any subsurface sand structure such as a "plenum" or shelf. In fact such structures will reduce the sediment volume that is available for the bacteria.

To put a sand bed in an established tank, remove any gravel or crushed oral, and then add the new sand, about an inch at a time. Don't worry overly much about the cloudiness in the water, reef animals are adapted to this and will be able to tolerate it easily. In a new tank, simply put all of the sand you need on the tank's bottom, add water and place your live rock in or on the sand. There is no need for any sort of platform to support the rock. I embed the live rock a bit into the sediment to give it stability.

After filling the tank with water, some initial bacterial and infaunal source will need to be added. This is usually "live sand" or "LIVE SAND." The former is simply wet sand that has been in a marine system at some time. It will have some bacteria in it, but little else, and I consider it worthless. "LIVE SAND" will have been collected in a marine area and shipped directly to you or the pet store where you may purchase it. It often has a reasonable array of animals in it, and it is what you need to give your tank a good start. At least ten percent of your total tank volume, by weight, should be LIVE SAND; more is better.

Additionally, there are several vendors offering "detritivore" or "recharge" kits having several different types of animals in them. Kits from different vendors are complementary rather than competitive. Adding one kit is good, adding two is really better. You will get a more diverse system faster with more kits. However, their cost may be prohibitive. Once the kits are in, let the system go for at least two weeks without adding fish to allow the live sand animals to establish minimal populations. Remember these are living animals, and will need to be fed.

Left. Fireworms such as this Eurythoe complanata , are among the most desireable of the animals added in "detritivore kits." Center. Gammarid amphiphods are great detritivore/scavengers and are also commonly sold in faunal "kits." Right.Nassarius snails are very good scavengers of meat or meaty foods and are harmless to all healthy animals in a tank. They need to have sand surface to bury into, and should be added at the rate of two to five per square foot of sediment surface.

Within a week, you should notice bubbles in the sediment next to the glass indicating the sand filter is working, within a couple weeks small tube traces should be visible in places in the sediments near the walls, and small bug populations should be evident. After a two week wait - and more time is desirable - fish may be added. UNDER NO CIRCUMSTANCES SHOULD YOU ADD "SAND-SIFTING" ANIMALS SUCH AS BURROWING SEA STARS OR SOME GOBIES. These animals are "sifting" the sediment to eat the sand critters that you need to have thrive. From this point, gradually add more animals up to the desired level.

Problems:

More imagined than real problems bedevil keepers of sand beds. The imagined problems are proposed by people who are ignorant of the sand bed dynamics. Among these imaginary problems are accumulations of hydrogen sulfide and detritus, and the need for sifting. Hydrogen sulfide will indeed be formed in the lowermost layers of a deep sand bed. It will NOT migrate up through the sediments to poison a tank. Hydrogen sulfide is an amazingly toxic gas, but that toxicity is exceeded by its pungent rotten-egg odor. The gas will have an exceptionally strong odor, and will seem overwhelming at levels well BELOW toxic amounts. If you can smell this stuff without it literally taking your breath away, it won't be at a harmful concentration. There is no real evidence to indicate that it may reach toxic levels in a deep sand bed.

Detritus build up in the sediment is another non-problem. If the sediment fauna is thriving, there will be a slight build up of fine detritus while the rest will be processed by the infauna. The final imaginary problem, the presumed need for sifting in a healthy sand bed, simply does not exist. Small organism movements "sift" the sand sufficiently. Any other sifting of a healthy bed will cause serious harm.

Sand beds recycle materials and export many of the excess nutrients in an aquarium. Some excess nutrients are mobilized by becoming soluble through metabolic processes and need to be exported either as harvestable macroalgae or animals, grown in the main tank or a sump.

The only real problem with a sand bed is the reduction in diversity as the bed ages. This is caused by extinction and replacement problems because the volume of our beds is simply too small for some species to generate self-sustaining populations. This is remedied, by purchasing a detritivore or recharge kit or two every year or so to give a boost to the fauna.

Conclusion:

The installation of a live sand bed is easy, straight-forward, and inexpensive relative to almost all other aquarium purchases. Once established, such a bed will contribute much to the success of a reef tank by providing a biological filter with sufficient capacity to for most tanks. Additionally it will provide food for many of the suspension feeding animals such as small polyped stony corals. And, it will do this all with a minimum of care and expense.